The invention relates to an electronic device comprising a substrate having a first side, which substrate is provided at said first side with a first layer, which first layer comprises an electrically conducting portion and adjoining thereto an electrically insulating portion.
The invention also relates to a method of manufacturing an electronic device comprising the step of patterning a first layer in accordance with a chosen pattern, which first layer lies on a substrate and comprises an electrically conducting portion and an electrically insulating portion after patterning.
Such a device is known from U.S. Pat. No. 5,736,452. The first layer comprises a pattern of electrically conducting portions of aluminum which are separated from one another by electrically insulating portions of an electrically insulating material. The first layer in the device is manufactured in a usual manner, i.e. by the following steps: deposition of inter alia aluminum; provision of a layer of photoresist thereon; exposure of the photoresist in accordance with the chosen pattern through a photolithographic mask made for the purpose; development of the photoresist; removal by etching of the uncovered aluminum with any remaining photoresist thereon; removal of remaining photoresist from the etched aluminum; and deposition of the electrically insulating material on the etched aluminum.
It is a disadvantage of the known device that its manufacture is cumbersome.
It is a first object of the invention to provide an electronic device of the kind mentioned in the opening paragraph whose first layer is easy to manufacture.
It is a second object of the invention to provide a method of the kind mentioned in the opening paragraph in which the use of a photoresist is not necessary.
According to the invention, the first object is achieved in that:
the first layer comprises a material which is built up from at least a first and a second element,
the material has an amorphous state and a crystalline state,
the electrically insulating portion comprises the material in its amorphous state, and
the electrically conducting portion comprises the material in its crystalline state.
Both the amorphous and the crystalline state of the material are utilized in the electronic device according to the invention. The material is present in its crystalline state in the electrically conducting portions of the first layer. The material in the amorphous state acts as an electrical insulation between the electrically conducting portions of the first layer. The material in its crystalline state has in fact a better electrical conductivityxe2x80x94i.e. a lower resistivityxe2x80x94than in its amorphous state. Preferably, the conductivity in the crystalline state is more than 104 times that in the amorphous state.
In its amorphous state, moreover, the material may act as an electrical insulation between conductor tracks outside the first layer, which conductor tracks are as yet not electrically interconnected. At any moment after the manufacture of the device according to the invention, a transition from the amorphous to the crystalline state may be achieved locally in the material. This gives rise to one or several electrically conducting portions which electrically interconnect the conductor tracks outside the first layer. The transition from the amorphous to the crystalline state may be achieved in a portion of the first layer through heating of this portion. The transition remains limited to the heated portion.
The first layer provided with the electrically conducting and the electrically insulating portions may be readily manufactured in that the first layer of the material is deposited in the amorphous state and is subsequently patterned through local heating of the material in accordance with the chosen pattern.
The first layer of the device according to the invention may lie directly on the substrate, or alternatively it may be present as an intermediate layer in the device, or at a surface of the device. The device is, for example, an integrated circuit, a display, a filter, an optoelectronic device, or a network of passive components.
Phys. Rev. Letters 21 (1968), 1450-1453 by S. R. Ovshinsky discloses a device comprising a material in which a transition from an amorphous state to a crystalline state of the material is utilized for switching and for making a memory unit. In the device according to the invention, crystalline and electrically conducting portions are present in a largely amorphous and electrically insulating layer. Whereas the material in the known device comprises at least the atoms of tellurium or arsenic, there is no need for this in the device according to the invention.
A number of materials are known from EP-A-0 644 537 in which a transition from an amorphous state to a crystalline state is achieved through irradiation with a laser beam. A difference in optical reflection is achieved thereby, which is utilized for data storage. It is not known or even suggested therein that such materials can be used in alternative applications on account of their difference in conductivity. Applications in which this difference in conductivity is utilized, such as the device according to the invention, are not demonstrated.
The material aluminum-germanium is known from Catalina et al., Thin Solid Films 167 (1988), 57-65. This article, however, does not show or suggest the use of the material in an electronic device. The article contains no suggestion on patterning of a first layer through local heating of the aluminum-germanium.
It was surprisingly found in experiments which led to the invention that several atoms may be chosen for the first element of the material, and that the second element may be chosen from gallium, germanium, and indium. If silicon is chosen as the second element, a slight drop in the resistivity is found, such as with Cr-Si, or the amorphous state is found to be instable, as is the case for Al-Si. Preferably, the proportional quantity of the second element in the material is at least 10%.
Examples of first elements are inter alia vanadium, cobalt, nickel, copper, zinc, gallium, aluminum, silver, tellurium, zirconium, titanium, molybdenum, antimony, arsenic, and tungsten. The first and the second element are different here. Preferably, the proportion of the first atoms type in the material exceeds 30%. Examples of first materials are Al-Ge, Ge-Te, Ga-Sb, and In-Sb.
The material may comprise a third element. The inclusion of the third element in the material may raise the resistivity in the amorphous state.
It is favorable when the first element of the material in the first layer is aluminum, and the second element is germanium, the germanium content being at least 20%. The crystalline state of the material aluminum-germanium has two phases: a phase of substantially pure crystalline aluminum and a phase of substantially pure crystalline germanium. Examples of favorable compositions of the aluminum-germanium are shown in Table 1.
A first advantage of aluminum-germanium as the material is that a conductivity was found for the material in its crystalline state which is only ten times smaller than that of pure aluminum. The inventors are of the opinion that this is caused by the presence of a substantial proportion of a crystalline phase of aluminum. A second advantage of the aluminum-germanium is that the crystallization temperature lies between 120 and 250xc2x0 C. This is a temperature which can be readily achieved by local heating with a laser beam. At the same time, this temperature is high enough for preventing a spontaneous transition from the amorphous to the crystalline state. A third advantage is that the elements of aluminum and germanium are known per se in the manufacture of, for example, semiconductor devices. The purchase price of aluminum and germanium is comparatively low. A fourth advantage is that aluminum and germanium are easy to process. These substances are not toxic or reactive in combination with, for example, oxygen and water.
In a first embodiment of the device according to the invention, a first and a second conductor track are present, and an electrically conducting portion of the first layer extends from the first to the second conductor track. The electrically conducting portion in this embodiment is an interconnect between the first and the second conductor track. The first portion may be an interconnect which provides a lateral connection; in that case the first portion comprises an extension from a first to a second point in the first layer. Alternatively, the first portion may be an interconnect which provides a vertical connection; in that case the first portion comprises an extension transverse to the first layer. Such an interconnect is known to those skilled in the art as a via. The conductor tracks themselves in the device may be, for example, an interconnect or an electrode of a transistor, of a capacitor, or of a resistor.
A first advantage of this embodiment is that a further interconnect can be manufactured without the necessity of making a new photolithographic mask. The adaptation of a pattern of interconnects is easy. An interconnect, once made, may be removed by means of laser ablation. A second advantage of this embodiment is that the first layer can be used as an auxiliary layer in addition to a normal interconnect. If the normal interconnect should fail to function, the device can be repaired in that the first portion of the first layer is crystallized.
In a first modification of the first embodiment, the electrically conducting portion of the first layer is covered with a second layer of electrically conducting material. The second layer is preferably provided by means of electrochemical deposition. For example, the second layer has a thickness of a few micrometers, and a coil is defined in the second layer. The second layer may alternatively comprise an end contact or a bond pad.
In a specific modification, the first layer has a relief structure. The first insulating portion is partly removed from the first layer. As a result, the substrate is free from a covering first layer at the first side at the area of a removed portion. The removal of portions may take place, for example, by laser lithography or by etching. It is an advantage of this modification that undesirable currents through the electrically insulating portion of the first layer are prevented. A current through the first, interposed, electrically insulating portion may be substantial, especially if the first and a second electrically conducting portion in the first layer lie approximately parallel at a short distance from one another. If the first layer has a relief structurexe2x80x94the electrically insulating portion being at least partly absent between the first and the second conductive portionxe2x80x94, this current will be substantially absent. An application of this embodiment is, for example, a polymeric substratexe2x80x94a printed circuit boardxe2x80x94on which the first layer and a number of components are present.
In another modification of the first embodiment
the first layer has a first and a second side, which sides face away from one another and are at least partly parallel to the first side of the substrate,
the first conductor track is situated at the first side of the first layer, and
the second conductor track is situated at the second side of the first layer.
In this modification, the first portion of the first layer may be a vertical interconnect area or via. The second electrically conducting pattern may alternatively be an electroplated enhancement of the electrically conducting portion of the first layer. The achieved advantage of an easy manufacture is particularly apparent in the case of a via in the first layer. Not only is the manufacture of the via in the first layer limited to one step instead of a large number of steps, but in addition a planarizing step is unnecessary. The risk of leakage currents between adjoining vias in the first layer can be prevented substantially completely in that the first layer is given a small thickness; preferably, the thickness of the first layer is smaller than 100 nm, and more preferably smaller than 50 nm. The decrease in thickness causes the resistance value of the via between the first and the second conductor track to decrease, while at the same time the resistance value for the leakage between adjoining vias increases.
In a second embodiment of the device according to the invention:
a first component with a first conductor track for external contacting is present on the substrate,
said first component is hermetically enclosed in an envelope having an outer side, which envelope comprises the first layer and the substrate, and
the first conductor track of the first component can be contacted at the outer side of the envelope via the electrically conducting portion of the first layer.
In this embodiment, the first layer is a protective layer of the device, the electrically conducting portion being a connection surface for external contacting. The first component may be accommodated on the substrate, or alternatively it may be manufactured on the substrate through deposition of consecutive layers. Besides the first component, a plurality of components may be hermetically enclosed by the first layer and the substrate. If so desired, the electrically conducting portion at the outer side of the envelope may be electrochemically enhanced with a second pattern. It is furthermore possible for the first portion of the first layer to extend from the first side to a second side of the substrate, which second side faces away from the first side. The first portion, possibly with its electrochemical enhancement, then is a U-shaped connection surface. This has the advantage that the device is a Surface Mountable Device.
The use of the first layer as a protective layer is advantageous. Such a protective layer is known to those skilled in the art as an electronic package which provides a hermetic sealing of an enclosed component. A first advantage is the simple manner in which electrically conducting portions can be provided: first the first layer is provided, and then a transition to the crystalline state is achieved in a first portion which adjoins the first conductor track. A second advantage is that the package of the material provides not only a good mechanical protection, but also protects against electrostatic discharges and corrosion. A metal layer is impermeable to gases and water. A third advantage is that electrically conducting portions can be provided in the first layer of the material also after the manufacture of the device has been completed. The provision of electrically conducting portions may take place, for example, during the mounting process of the electronic device on a printed circuit board.
The second object relating to a method of the kind mentioned in the preamble is achieved in that
the first layer comprises a material which is built up from at least a first and a second element, and which has an amorphous state and a crystalline state,
the first layer is heated in accordance with the chosen pattern up to at least a crystallization temperature of the material, whereupon the material is present in its crystalline state in the patterned electrically conducting portion and in the amorphous state in the electrically insulating portion.
After the first layer has been provided, a pattern of electrically conducting portions can be manufactured in an electrically insulating layer in a single step by the method according to the invention. The method is simple. The method is environmentally friendly because the use of photoresists and etchants for patterning the layer is unnecessary. Furthermore, the method is inexpensive.
An advantage of the method according to the invention is that a pattern of electrically conducting portions can be modified. Thus defects in patterning and design defects can be remedied, so that yield losses are prevented. It is also possible to provide the pattern of electrically conducting portions after the manufacture of an electronic device comprising the first layer has been completed. Changes in a design or in components of the device can thus be better accommodated. Portions brought into the crystalline state may be removed by laser ablation, among other means.
Possible means for heating up to at least a crystallization temperature include a focused electron beam, a needle-shaped heating source, a focused light beam, a laser beam, and laser lithography. The crystallization temperature usually lies between 100 and 400xc2x0 C., depending on the composition of the material. The heating up to at least the crystallization temperature is preferably carried out in that the layer is locally irradiated with a laser beam. Among the advantages of the use of a laser beam is that details of patterns can be small. A laser beam, moreover, is a known tool. Laser ablation of the material is also possible with a laser beam. The formation of a relief structure in the first layer is advantageous for preventing leakage currents between adjoining conductive portions in the first layer across interposed electrically insulating portions of the first layer.
In an embodiment, a second patterned electrically conducting layer is electrochemically provided on the first layer. The difference in conductivity between the electrically insulating and electrically conducting portions of the first layer causes the second layer to be deposited on and adjacent the electrically conducting portions. Electrochemical enhancement of the electrically conducting portions of the first layer with, for example, copper or gold is favorable for numerous applications in electronic devices. Examples of applications are end contacts, conductor tracks which can be used for strong currents and high frequencies, and thick conductor tracks which can be used as coils.
In a specific embodiment, the electrically insulating portion of the first layer is at least partly removed by etching, for which the second layer serves as an etching mask. Several etching methods are known to those skilled in the art, such as wet chemical etching and dry etching, anisotropic etching and preferential etching. An etchant which may be used for wet etching of aluminum-germanium is a mixture of iron trichloride hexahydrate and hydrogen chloride. Etching has the result that the electrically insulating portion of the first layer has a lesser surface area. Leakage currents across the electrically insulating portion can be prevented thereby.